The present invention relates to Doppler radar systems, and more particularly to an automatic thresholding circuit for Doppler radar systems.
The type of target to be tracked by a system which incorporates this invention has a "line-like" Doppler frequency spectrum. For example, for aircraft targets, the energy of the target returns is divided between the skin line and a series of upper and lower sideband lines generated by engine modulations. These upper and lower sideband lines may have more amplitude, for short periods, than the skin line.
One way of suppressing false alarms is to employ some type of automatic thresholding. Automatic thresholding in pulse doppler radars is done in several ways. One of the most common methods is to provide a bank of doppler filters to cover the frequencies between the pulse repetition frequency (PRF) lines. Detectors follow each doppler filter. The signal levels are compared against a threshold. If the signal level is greater than the threshold then the signal is detected. This threshold level can be generated by averaging the total power in the pass band between the PRF lines or by averaging noise power for a subgroup of the doppler filters.
Another common method uses returns from neighboring range bins about the range bin to be examined. These returns are processed individually as in the previous method and then added to form a threshold.
These methods are practical where component and processing complexity can be tolerated. Depending upon the actual implementation, these thresholding methods may not be very sensitive to variations in noise power in the intra PRF line frequency region caused by clutter and transmitter noise.